Emissions of pollutant chemicals have increased orders of magnitudes in the past 100 years due primarily to anthropogenic releases associated with industrial, agricultural, domestic, and recreational activity. Current research indicates that there are very strong correlations between the increase in these emissions and an overall increase in atmospheric temperatures (Global Warming) and an increased number of Category 4 and 5 hurricanes per annum. Furthermore, it is believed that ambient particulate matter in aerosol phase may include potentially toxic components. Researchers believe that particulate matter and gases may cause various health problems, such as asthma. The correlations between emissions of pollutant chemicals and the negative impact on environment and human health has led to more stringent worldwide emission standards.
In order to meet the emission standards of today and the future, researchers have made, and are continually striving to make, improvements to combustion engines, for example heavy duty diesel engines, gas combustion engines, power-generating gas turbines, and the like, and other emission sources. In addition to these developments, researchers are endeavoring for better methods and devices of measuring smaller particulate matter and quantifying the chemical compositions of trace emissions.
Generally, chemical composition analysis of fine particulate matter, inorganic gases, and volatile and semi-volatile organic compounds from emissions sources comprises three major steps: (1) Representative conditioning and sampling; (2) Chemical analysis; and (3) Data analysis and explanation. The effective accuracies of Steps (2) and (3) are both dependent on step (1). For without an accurate and precise sampling procedure, no analysis of that sample could be said to represent valid data. Accordingly, without valid analysis, full and complete explanation of the sample would not be available.
In collecting emission source samples, it is known to introduce a dilution gas, usually conditioned air (particulate matter, humidity, temperature, and gases controlled air), into the emission gas in order to dilute and cool the emission gas to near ambient conditions. This is intended to permit the sample gases to nucleate, condense and coagulate, and to be aged in a residence time chamber to their usual phases and conditions as if they were emitted to the atmosphere. For example, see L. M. Hildeman, G. R. Cass, and G. R. Markowski, “A Dilution Stack Sampler For Collection of Organic Aerosol Emissions: Design, Characterization and Field Tests”, Aerosol Science and Technology, Vol. 10, pp. 193-204, 1989.
In the United States, the typical system for assessing particulate matter mass emissions mixes emission gas with filtered air in a mixing chamber. The typical system is illustrated in FIG. 1, and includes a sampling port 2 that feeds exhaust gases to a diluter 4, forming the mixing chamber, where the exhaust gases are diluted with the filtered air. The diluted gas mixture is then sampled by a sampling train 6 to collect particulate matter mass. However, this typical system doesn't minimize a temperature gradient between sample gases and the inner wall of the mixing chamber and therefore may cause significant loss of sample particles during the dilution processes.
Work at the University of Wisconsin-Madison attempted to improve the traditional system. The University of Wisconsin scientists used a device called an “augmented sampling system” to study the chemical composition and to assess particle size of diesel engine exhaust. (Chol-Bum Kweon, David E. Foster, James J. Schauer, and Shusuke Okada, “Detailed Chemical Composition and Particle Size Assessment of Diesel Engine Exhaust” SAE 2002-01-2670, Fall SAE Meeting 2002) The “augmented sampling system” disclosed by Kweon et al includes a secondary dilution tunnel for the diesel exhaust and a residence time chamber with radial sampling ports near the base of the residence time chamber. The secondary dilution tunnel of the augmented sampling system mixes filtered air with an emission gas sample without regard to temperature gradient between the surface of the dilution tunnel and the emission gas. This may lead to a high degree of particle loss and accordingly less accurate sampling due to thermophoresis.
Thermophoresis, or Ludwig-Soret effect, is thought to be related to Brownian movement biased by a temperature gradient. The thermophoretic force is a force that arises from asymmetrical interactions of a particle with the surrounding gas molecules due to a temperature gradient. Generally, a particle is repelled from a hotter surface and attracted to a cooler surface. Thus, as emission particles travel through a sampling system, cooler surface temperature of the system as compared to the emission gas would lead to greater attraction on the emission particles.
In the Kweon et al. augmented sampling system, the residence time chamber is heated to reduce thermophoresis. However, the heated residence time chamber is likely to fail in simulating realistic atmospheric conditions, as the addition of heat may affect the aging, nucleation, condensation, and coagulation processes of particulate matter, volatile organic compounds and semi-volatile organic compounds and the secondary reaction of inorganic and organic compounds.
An apparatus that allows for mixing of sampled emission gas with conditioned air to dilute the emission sample and reduce particle loss due to thermophoresis is needed. An improvement in the diluting process is desirable because it may lead to a more accurate and precise sampling from the sample source and thus contribute to more accurate results.